**Author details**

292 Recent Advances in Crystallography

only the *5d5/2*

Contours as in Fig. 13.

*K K K K*

298 0.77(3) 0.5

**7. Bright future for X-ray crystallography** 

*8* are occupied. Since *5d5/2*

reporting that *5d-eg* and *2p* of B consist of the conduction band.

The electron configuration in Table 3 is correlated to the physical properties of SmB6 as follows: (a) SmB6 is a Kondo insulator. Its electric resistivity increases gradually like semiconductors below room temperature and begins to increase steeply below 30 K with a decrease in temperature. It also begins to increase like metals above room temperature (Ueda & Onuki, 1998). *2p* and *5d3/2Γ8* orbitals consist of the conduction band (Kimura et al., 1990). Since B-*2pz* extends along the edge, it does not overlap with *5d3/2Γ<sup>8</sup>* effectively and does not seem to contribute to the band. The population of *2px* in Table 3 as well as the resistivity steadily increases on lowering temperature. n(*5d5/2Γ8)* also increases from 230 K. The increase of populations indicates that of localized electrons. It may be correlated to the increase of the resistivity. (b) *4f7/2Γ6* are vacant only at 100 K, although they are occupied at the other three temperatures. It may be correlated to the band gap between the *4f* states, which is reported to start developing between 150 and 100 K (Souma, et al., 2002). (c) Among the *5d* orbitals

**Figure 14.** Difference density at 230 K after (a)spherical-atom refinement and (b) XAO analysis.

**Table 3.** Temperature dependence of electron populations of Sm *4f*, *5d* and B *2p* orbitals.

5/2 8 5/2 7 7/2 6 5/2 8 (4 ) (4 ) (4 ) (5 ) (2 ) (2 ) 100 0.96(9) 0.48(12) 0.0 1.0 0.21(8) 0..45(17) 165 1.0 0.67(13) 0.44(12) 0.75(24) 0.16(13) 0.55(39) 230 1.0 0.61(13) 0.59(13) 0.56(16) 0.13(7) 0.66(28)

9(7) 0.47(6) 0.78(6) 0.07(3) 0.81(12)

*x z n f n f n f nd np np*

EDD investigation was limited up to 3*d*-transition-metal complexes. However, XAO analysis made EDD investigations of rare-earth compounds as well as non-stoichiometric ones possible. Its application to organic compounds can be attained when it is developed to X-ray

as illustrated in Fig. 1, it agrees with the band calculation of LaB6 by Harima (1988),

*<sup>8</sup>* orbitals correspond to eg in the strong field model

Kiyoaki Tanaka and Yasuyuki Takenaka *Nagoya Institute of Technology, Hokkaido University of Education, Japan* 
